Note, this isn't an issue of changing one's mind the night after. This would be the same as in the US, if you asked your partner "do you have AIDS" and they say "no" and you later found out they had it and knew it, Is that rape? Not in the US, but because AIDS is deadly, it has been pursued as a criminal offense.
That's the closest US analog I can come up with.
That's part of the trouble, analogs that don't completely apply, combined with allegations that can't be proven.
Every so many months there are fairly high-profile rape or other sexual abuse violations in the news. A common thread in most of these situations is that they lack proof, and usually it's due to action or inaction on the part of the accuser. Some cases find the accuser's actions like bathing or waiting too long to have destroyed physical evidence, and other cases find documentation showing the accuser continuing to pursue friendship or sexual contact with the accused, and these are in places where the law doesn't place a legal distinction in one partner following another partner's directions or mandates before the consensual act happens. Sweden's rules that allow partners to define the conditions under which one partner consents that are binding but not documented before the act itself make it very difficult to determine what actually happened, and as in other high profile cases it sounds like there's evidence of continued contact after the act that was eventually the basis of the complaint.
If I understand the law where I live, once the choice to commence sexual intercourse is made, so long as the acts themselves are, "natural," there is no legal basis for conditional consent based on the use of contraception or lack thereof by one party or the other. The act itself is assumed to carry the responsibility that it could lead to pregnancy or other undesired ramifications even if contraception or other forms of protection are employed as that is the point of sex to begin with, so the responsibility to use protection is borne by each individual in the act for themselves, rather than on the other person, and that the ramifications for the act are strictly personal outside of the realm of conception and shared responsibility to any offspring. In short, anyone that voluntarily engages in natural sexual congress is strictly responsible for themselves and for any progeny as a result, and that the other person, within certain bounds (like your reference to knowingly lying about some fatal infectious diseases) is not responsible for the other person's well-being.
Assange might be a horrible person, convincing women to have riskier sex with him than they otherwise would normally agree to, but between the difficult nature of quantifying responsibility in an otherwise consensual sexual encounter and a lack of definitive evidence that the offense actually occurred coupled with contrary evidence that indicates a cordial relationship post-act, it's difficult to understand how there's enough to make an international case of it. After all, history is full of charismatic seducers that convinced others to have sex with them despite their original antipathy without labeling the act as a form of rape or other illegal sexual act. Calling Assange's act criminal should mean that the act of women intentionally ceasing the use of birth control to get pregnant without communicating with their partners is also criminal. It's not wise to define either that way though, and since there isn't a way to prove allegations without an admission, it makes more sense to leave the burden of responsibility on each person, for themselves, when they agree to commence the act itself.